1. Field of the Invention (Technical Field)
The present invention relates to fuel loading of vehicles and more specifically to a method and apparatus for determining a specific fuel amount required for a number of variable conditions and an apparatus for accurately delivering the predetermined fuel amount to the vehicle.
2. Background Art
The arena of unmanned aerial vehicles (UAV's) includes many types of vehicles performing many types of missions. Various fixed wing UAV's (FWUAV's) and vertical take off and landing UAV's (VTOLUAV's) exist and continue to emerge. UAV's exist in a variety of sizes from full-scale airplanes to micro aerial vehicles (MAV's), which can be as small as a few inches along their major axis. MAV's exist in fixed wing configurations (FWMAV's) and vertical take off and landing configurations (VTOLMAV's).
VTOLUAV's require more fuel-burn per unit payload-hour than do FWUAV's. VTOLMAV's require even more fuel-burn per unit payload-hour than do the larger VTOLUAV's. VTOLMAV's in particular are challenging the design parametric trade for optimization of total wet vehicle weight, payload weight, environmental conditions and time aloft or mission duration.
The design trade space consists of some key parameters that must be considered for variability. The first parameter is the environmental operational envelope variability in air density due to temperature and altitude. These conditions can vary from sea level standard day (SLSD) to 12K ft. altitude hot day for UAV's flying at low above ground level (AGL) missions to above 40K ft. altitude for UAV's flying high AGL missions. Another important parameter is the environmental operational envelope variability due to cross/gusting and head/tail wind conditions, and the changes in AGL that the. UAV is exposed to during a mission. Of course, the other key parameters are the mission duration or the desired time aloft for the UAV and the payload weight.
Consideration of all of the above result in a specific and unique fuel loading or “wet” vehicle weight for each of many conceivable mission scenarios. Many different UAV's can be designed, each of which to satisfy a specific combination of the mission parameters of environmental envelope, duration and payload. The principal difference between these various possible vehicles is the size of the fuel tank.
Maximization of both payload capability and mission duration in UAV's is of great interest. The maximization of these two operational parameters is performed at a particular vehicle design-point condition of altitude, temperature, and resulting total vehicle wet weight. The UAV's fuel is accordingly sized. If sea level standard day (SLSD) is selected for the design point and the parameters optimized, the UAV may not be able to operate in the degraded performance conditions of higher altitude (i.e.: for the selected SLSD design point weight and mission duration the UAV may not have enough power to lift off at higher altitudes.)
Conversely, if a high altitude and standard temperature are selected for the UAV's design point, the UAV will have unexploited capacity (in terms of additional payload weight and/or additional mission duration or time aloft) when it is operated at SLSD conditions. The UAV may be loaded with greater weight payload, but the conventional design-point specific approach will not allow additional fuel for greater time aloft at the SLSD condition.
In order to have one vehicle designed to meet the myriad of combined mission parameter possibilities there is a need to be able to quickly fuel load UAV's to differing fuel amounts that correspond to each unique mission need of environmental envelope, duration and payload. This results in a specific wet vehicle weight and a corresponding desired fuel load, which is specific to each unique mission. Also, in order to quickly conduct the refueling to a specific fuel level it must be performed independent of knowing the level of any residual fuel remaining in the fuel tank from any prior mission or use.
Another important consideration encountered with the military application of UAV's is that the vehicles may be launched by soldiers that are in harms way. Time may often be of the essence when refueling a UAV for launch. Further, the fueling system must be lightweight and simple for field use.
There are several conventional methods that are currently available, however their shortcomings are obvious. A dip-stick can be utilized, which is a trial and error method consisting of check-fill-check-repeat. Overfilling is a danger with this process and recovering from an overfill condition is difficult, time consuming and messy. Another device is a sight glass. These devices come in various types, but they all add weight and complexity to the vehicle. Weight is a paramount premium in UAV design. Manual fluid measurement prior to filling have also been used. Prior to initiating the refueling process with this method there is a need to know the amount of residual fuel remaining in the tank, which requires dip-sticking or some other supporting method all of which is time consuming. On-board fuel level indicators also exist consisting of various mechanical float and/or electromechanical devices (i.e.: gas gauge as in a typical automobile) all of which add weight and complexity to the vehicle.
All of the above mentioned conventional methods add weight to the air vehicle and/or require time via a concerted effort to fuel the vehicle a predetermined amount. Most of the conventional methods preclude the ability to rapidly fuel the vehicle with a predetermined amount of fuel.